it used to raise an error while it should return None

core: add child handler. WARNING: libev reaps all child processes now. to disable it, pass 'nochild' option to loop.

remove baseserver.DEFAULT_MAX_ACCEPT variable; add get_listener() class methods on StreamServer and DatagramServer

it was not enabled in patched tests because monkey.patch_all() imports threading and that prevents Hub from being patched with CountingHub

spawn manager greenlet if necessary to finish off excess threads

test__greenness.py: import greentest before monkey patching, since patch_all() installs hub and greentest monkey patches Hub class

make Pool's add() and discard methods usable on their own. Based on pull request #10 (https://bitbucket.org/denis/gevent/pull-request/10/fix-2-bugs-in-pool-class) by Danil Eremeev.

core: always pass EVFLAG_NOENV to libev's loop. LIBEV_FLAGS env var is now ignored (use GEVENT_BACKEND)

python's threading module relies on the lock's class __exit__ to not
call into the tracing function set via sys.settrace until the lock
really is released. The only way to prevent this is to implement it in
C. So, we move coros.Semaphore to a cython module.

fixes the failing tests in test__threading_vs_settrace.py

explicitly test that sys.settrace isn't called and test the same
issue when a non-main thread exits.